Photoreactivity of the sunscreen butylmethoxydibenzoylmethane (DBM) under various experimental conditions

The photoreactivity of the sunscreen butylmethoxydibenzoylmethane (DBM) was studied in three specific environment types: (i) by irradiating diluted solutions in solvents of various polarities; (ii) by irradiating concentrated solutions in non-volatile solvents; (iii) by irradiating thinly applied commercial sun products. The behaviour of DBM clearly showed that its sensitivity to light is dependent on experimental conditions. DBM is stable in a polar solvent such as alcohol, whereas in a non-polar solvent, DBM is undergoing a reversible displacement of tautomeric equilibrium. This unexpected phenomenon is inhibited by a very small modification of polarity. The photodegradation products were detected using liquid chromatography–mass spectroscopy (LC/MS), to detect their molecular masses and thus obtain a broad idea of their identity. The incidence of photodegradation on the sun protection factor (SPF) value was measured. Because the degradation of DBM appears unpredictable, given that sunscreen products are marketed as supplementary health protectors against pathologies related to the harmful effects of sun exposure, the photostability of sun products should be a major safety criterion. There are two reasons for concern: firstly, reduction of protection as a function of progressive irradiation and secondly, the appearance of photodegradation products on the skin surface.